1. Field of Invention
The present invention relates to a method of fabricating a sensor. More particularly, the present invention relates to a method for electrically connecting a photo-diode with a MOS gate.
2. Description of Related Art
Charge coupled devices (CCDs) are often employed in digital sensors for image extraction. Applications thereof include close-circuit TVs, cameras, and video recorders. However, CCDs are quite costly to produce and bulky. Hence, in order to reduce volume, energy consumption and cost, CMOS photo-diodes that can be formed by semiconductor techniques are a major future substitute for CCDs.
A sensor often comprises a circuit area, and a photo-sensitivity area composed of a photo-diode. The photo-diode is a light-sensitive (or light-detecting) semiconductor device that converts light energy into electrical signal through a P-N junction. Due to the existence of an internal electric field at the P-N junction, electrons in the N-doped region and holes in the P-doped region cannot diffuse across the P-N junction when no light shines on the junction. However, when a light beam of sufficient intensity impinges upon the junction region, electron-hole pairs are generated within the junction region. These electron-hole pairs, while reaching the region with an internal electric field, separate from each other. The electrons migrate towards the N-doped region while the holes migrate towards the P-doped region, thereby leading to a current flowing in the P-N junction electrodes. Ideally, the photo-diode should be in open-circuit condition having no electric current flowing when the device is in the dark.
Conventionally, the signal is transmitted from the photo-sensitivity area to the circuit area through only a metal line connecting with the P-N photo-diode. A junction between the metal line and the P-N photo-diode has a very low potential barrier. If a dark current exists in the P-N photo-diode, it flows through the junction barrier to the metal line easily, and becomes a noise signal causing data misjudgment,
The present invention provides a method of fabricating a MOS sensor. A P-doped region extending into a substrate is formed. A stacked polysilicon structure is formed over the P-doped region. Ions are implanted into the P-doped region to form an N-doped region extending into the substrate by using the stacked polysilicon structure as an implantation buffer layer. The stacked polysilicon structure is patterned and etched to form a stacked polysilicon ring over the N-doped region. A metal line is formed for electrically connecting the stacked polysilicon ring with a gate of a MOS transistor.
Preferably, the stacked polysilicon structure is formed by the steps of depositing a first polysilcon film over the P-doped region and then depositing a second polysilicon film over the first polysilicon film. Before the second polysilicon film is deposited, the first polysilicon film can be patterned and etched to form at least a window exposing the P-doped region. After this window formation, the second polysilicon film can be deposited to cover the first polysilicon film and to make contact with the P-doped region through the window.
The stacked polysilicon structure serves as an implantation buffer layer that protects the substrate from being damaged in the implantation step. Moreover, by electrically connecting the photodiode with the MOS gate through a stacked polysilicon ring and a metal line, the value of the dark current read out from the photo-diode is reduced when the MOS sensor is operated.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.